The reduced form of ascorbic acid in urine can falsely depress semiquantitative urine tests for glucose, hemoglobin, nitrite, etc. by reversing the oxidation of colored indicators. The reducing activity of reduced ascorbic acid can also falsely elevate the apparent glucose when using the clinitest tablet method. Our studies show that ingestion of modest amounts of vitamin C supplements is sufficient to cause such interference in urinary glucose determinations. This project is designed to study the extent to which this is a practical problem by measuring reduced ascorbic acid in subjects taking vitamin C supplements and relating the ascorbic acid concentration to the extent of interference with methods in use in our laboratory. Using a colorimetric method for measuring urinary ascorbic acid, we found that high urine concentrations could occur even with oral vitamin C supplements of 250-1000 mg per day. These ascorbic acid concentrations were sufficient to cause negative interference with dipstick assays for urine glucose and hemoglobin. The interference was related to the amount of ascorbic acid in the urine and to which dipstick was used.